Dynamics of Water Adsorption from Butanol–Water ... - ACS Publications

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Dynamics of Water Adsorption from ButanolWater Vapor in a Biosorbent Packed Column Qian Huang, Ajay K. Dalai, Jinho Park, Angie M Diaz, Ellen Hyeran Kang, and Catherine Hui Niu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03377 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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Industrial & Engineering Chemistry Research

Dynamics of Water Adsorption from Butanol-Water Vapor in a Biosorbent Packed Column Qian Huanga, Ajay Kumar Dalaia, Jinho Parkb, c, Angie M. Diazb, Hyeran Kangb, c, d, and Catherine Hui Niua aDepartment

of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan, S7N 5A9, Canada bNanoScience

Technology Center, University of Central Florida, Orlando, FL 32826,

USA cDepartment

of Materials Science and Engineering, University of Central Florida, Orlando, FL 32816, USA dDepartment

of Physics, University of Central Florida, Orlando, FL 32816, USA

Corresponding author. Address: Department of Chemical and Biological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan, Canada S7N5A9. Tel: 1 306 966 2174. Fax: 1 306 966 4777. E-mail address: [email protected] 

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Abstract Drying is essential in the production of bio-butanol. In this work, the dynamics of water adsorption from butanol-water vapor mixtures in a fixed bed column was investigated. The biosorbent used in this system was derived from a cellulosic material oat hull. Water adsorption breakthrough curves obtained from the biosorbent packed column were simulated by the Bohart–Adams model and Klinkenberg model. The Klinkenberg model provided a better simulation of the experimental data in comparison with the Bohart–Adams model. The mass transfer coefficient and mass transfer resistances were investigated from the modeling results. The results indicated that the rate of water adsorption was controlled by internal mass transfer resistance. And the water adsorption on the biosorbent was visualized in aid of microscope imaging. This study reveals that water saturated oat hull based biosorbent was regenerated and reused successfully.

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Abbreviations ABE

Acetone 1-butanol and ethanol

BET

Brunner-Emmet-Teller

FTIR

Fourier transform infrared spectroscopy

LDF

Linear driving force

RSS

Residual sum of squares

SEM

Scanning electron microscopy

TGA

Thermogravimetric analysis

List of symbols c

Water concentration, mol/m3

c0

Water concentration in the feed, mol/m3

cef

Water concentration in the effluent, mol/m3

De

Effective diffusivity, m2∙s-1

Di

Molecular diffusivity, m2∙s-1

DL

Axial dispersion coefficient, m2∙s-1

Dp

The diameter of adsorbent particles, m

kBA

Bohart–Adams rate constant, m3∙kg-1∙min-1

kc

External mass transfer coefficient, m/s

kLDF

Overall mass transfer coefficient, min-1

K

Adsorption equilibrium constant

𝑞

Average water adsorption capacity, mol/m3

qs

Saturated adsorbate loading in adsorbent, mol/m3

3



Re

Reynolds number

Rp

The radius of the adsorbent particles, m

R2

Correlation coefficient

Sc

Schmidt number

Sh

Sherwood number

t

Time, s

T

Temperature, °C

u

Interstitial velocity, m/s

z

Distance from the bed entrance, m

Z

Length of the column, m

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Greek letters εb

Bed porosity

ξ

Dimensionless distance

τ

Dimensionless time

4


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1. Introduction The development of biofuels is increasingly important due to limited availability and rapid depletion of fossil fuels, the need of reduction in greenhouse gas emissions, and the possible increase in the price of crude oil.1 Biobutanol is a promising biofuel to be blended with gasoline due to its higher energy content, lower vapor pressure, lower flash point, and less corrosive character when compared with ethanol.2,3 Butanol can be used in butanol−gasoline blends without having major modifications in the engines.4 Biobutanol can be produced from acetone, 1-butanol, and ethanol (ABE) fermentation. In order to separate biobutanol from the fermentation broth, preliminary distillation is usually applied to get the azeotrope product of butanol (approximately 56 wt% butanol in water). Then, the decantation and multiple distillations are used for further purification,5 which is an energy-intensive and costly method. Among various alternative approaches for butanol recovery, adsorption-based recovery technologies have received considerable attention owing to their low cost, economic feasibility, and easy operation.6–9 Adsorption is widely used for gas separation and a variety of drying operations. Adsorption has a low energy requirement as compared to conventional separation method. For example, separation of ethanol-water azeotrope by adsorption has been applied to replace the conventional method in industry.10 Recently, the use of low-cost and biodegradable biomaterials (starch-based and cellulose-based natural materials) as adsorbents instead of traditional adsorbents (such as molecular sieves, silica gel, etc.) has attracted more attention. Biosorbents are attractive since they are more available, cost-effective and environmentally friendly than conventional adsorbents. Furthermore, they can be regenerated easily by heating with a 5



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lower regeneration temperature compared to conventional adsorbents such as molecular sieves, silica gels, calcium chloride and so on. The regeneration temperature of oat hull based biosorbent used in this study is 105 °C, which is lower than that (around 175-300 °C) of the above mentioned commercial adsorbents.11,12 It has been reported that, compared to other types adsorbents, the biosorbents were more energy efficient.13 Dehydration of butanol using biosorbent provides a promising method for bio-butanol purification. It is more cost-efficient and environmentally friendly than conventional distillation.14 Since then, more starch-based and lignocellulosic enriched natural biomaterials were studied for water adsorption and alcohols drying operation.15–18 Water adsorption using the bio-based adsorbent could be a promising method for butanol purification, which is more cost-efficient and more eco-friendly compared with conventional distillation.14 Oat hull, the agricultural by-product of oat, is a potential adsorbent for alcohols dehydration based on the fact that lignocellulosic materials can be potential alternatives for drying alcohols.19,20 In a previous study,21 the oat hull based biosorbent has been applied as a biosorbent for butanol dehydration. The results have demonstrated that the oat hull based biosorbent was able to effectively produce high purity butanol product from butanol and water mixture with a concentration range of 56-90 wt% butanol.21 This provides an alternative method to reduce or replace the multiple downstream decantation and distillation units following the preliminary distillation, which generates the 56 wt% butanol–water azeotrope vapor in the bio-butanol industry. However, the dynamics of water adsorption, the mass transfer resistance, and limited step during water adsorption on cellulosic materials such as oat hull are not well understood. Investigation in this area is essential to understand the fundamentals of water biosorption 6


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and apply it in the industry.22 Such work has not been reported yet. Theoretical modeling is a feasible way to study the dynamics of the adsorption process in fixed-bed columns. Many mathematical models have been used to evaluate the effects of the design variables on the adsorption performance in fixed-bed columns.23–27 This work aims to investigate the dynamics of water adsorption from water/butanol mixture using oat hull biosorbent in a packed column. The focuses are on the analysis of water breakthrough curves and mass transfer in aid of mathematical modeling. Two dynamic models were used. The first one is the Bohart and Adams model, which assumes that the diffusion steps are very fast, and the surface adsorption is the rate controlling step. The second one, Klinkenberg model, which is based on linear driving force (LDF) model and linear adsorption isotherm, assumes that mass transfer is the rate controlling step when the adsorption rate is fast. The fitting results for these two models were compared to study the adsorption dynamics and mechanisms. This study applied a method to estimate the overall, the external and the internal mass transfer resistances. It assisted in identifying the rate controlling step. Also, in this study, the effects of adsorption temperature and feed concentration on adsorption dynamics were discussed. Furthermore, the regeneration of water saturated biosorbent and the reusability were studied. 2. Dynamic models Mathematical models can be used to evaluate the behaviour of the fixed-bed column and the effects of the design variables. The practical adsorption processes in fixed bed are complex. Thus, it is very challenging to solve models taking into account the equilibrium, axial dispersion, and mass transfer.28 Reasonable simplification needs to be applied to

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provide a shortcut analytic expression of the models. The mass balance for the adsorption column can be described in a partial differential equation (eq 1). ∂𝑐

∂𝑐

∂𝑡 +𝑢∂𝑧 +

(1 ― 𝜀𝑏)∂𝑞 𝜀𝑏

∂2𝑐

∂𝑡 ― 𝐷𝐿∂𝑧2 = 0

(1)

The initial and boundary conditions are as follows:

q(0, z)=c(0, z)=0 (t=0) c(t, 0)=c0 (t>0) t is time; z is the distance from the bed entrance; c is the water concentration; c0 is water concentration in feed; εb is the void fraction of bed, which can be represented as the difference of the bed volume and packed biosorbent volume divided by the bed volume; u is the interstitial velocity, which is the superficial velocity divided by the void fraction of bed; 𝑞 is average water adsorption capacity; DL is the axial dispersion coefficient. In eq 1, the third term is the adsorption rate based on 𝑞.29 In this work, the temperature of the bed was controlled by the jacket of the column, the isothermal assumption was made. The mass balance equation for the adsorption column together with the initial and boundary conditions were used to simulate the dynamics of water adsorption in the fixed bed. The adsorption rate expression in eq 1 was determined by Bohart–Adams model and Klinkenberg model, respectively. 2.1 Bohart and Adams model The Bohart–Adams model is widely used to describe the breakthrough curves of fixed bed for biosorption research.30 This model assumes that the diffusion steps are very fast, and the rate is limited by surface interaction (adsorption).28 The analytical solution of the mass balance equation (eq 1) was derived by Bohart and Adams, and the Bohart–Adams model is given in eq 2.31,32 8


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(

𝑙𝑛

)

𝑐0

𝑐𝑒𝑓 ― 1 =

( )―𝑘

𝑘𝐵𝐴𝑞𝑠𝑍 1 ― 𝜀𝑏 𝑢

𝐵𝐴𝑐0𝑡

𝜀𝑏

(2)

where kBA is the Bohart–Adams rate constant; qs is the saturated adsorbate loading in adsorbent; cef is water concentration in the effluent; and Z is the length of the column. The parameters kBA and qs can be calculated from the slope and intercept by linear regression. This model was tested in this work to simulate water adsorption breakthrough curves in comparison with the one below. 2.2 Klinkenberg model The above Bohart–Adams model, which is based on the process controlled by the surface adsorption step, does not take into account the external and internal diffusion limitations in the overall process.28 Considering that the biosorbent is porous, the Klinkenberg model considering mass transfer limitation by diffusion is also considered in this work. It’s represented by linear driving force (LDF) approximation of mass transfer by diffusion. In addition, the linear adsorption isotherm is used to correlate water uptake to water concentration. In this model, the surface adsorption process is assumed to be much faster than both external and internal mass transfer resistances. An approximate solution of eq 1 was provided by Klinkenberg,33,34 which is presented below: 𝑐𝑒𝑓 𝑐0

[

1

≈ 2 1 + 𝑒𝑟𝑓 𝜉=

(

𝜏― 𝜉+

1 8

+8 𝜏

1

)]

𝜉

( )

𝑘𝐿𝐷𝐹𝐾𝑍 1 ― 𝜀𝑏 𝑢

𝜀𝑏

(

𝑍

𝜏 = 𝑘𝐿𝐷𝐹 𝑡 ― 𝑢

)

(3) (4) (5)

kLDF is the overall mass transfer coefficient; K is the adsorption equilibrium constant. erf(x) is the error function. ξ and τ are dimensionless distance and dimensionless time. The 9



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mass transfer coefficient kLDF can be determined through fitting the experimental water breakthrough curves to Klinkenberg model. The relationship of the overall mass transfer resistance with external and internal mass transfer resistances is shown in eq 6.28 𝑅2𝑝

𝑅𝑝

1 𝑘𝐿𝐷𝐹𝐾

(6)

= 3𝑘𝑐 + 15𝐷𝑒

𝑆ℎ = 2 + 1.1𝑅𝑒0.6𝑆𝑐 𝑆ℎ =

13

𝐷𝑝𝑘𝑐 𝐷𝑖

(7) (8)

kc is external mass transfer coefficient; De is effective diffusivity; Rp is the radius of the adsorbent particles; Di is molecular diffusivity. Dp is the average diameter of adsorbent particles, which is taken to be the equivalent of the volume median diameter of particles determined by particle size distribution in this work. In eq 6, the first term is overall mass transfer resistance, and the second and third terms are external and internal mass transfer resistances. According to eq 7 and eq 8,35 kc can be estimated from Sherwood number (Sh), Reynolds number (Re) and Schmidt number (Sc). In addition, once kc is obtained, the internal mass transfer resistance can be estimated by eq 6. 3. Materials and methods 3.1 Biosorbent and other materials In this work, the raw oat hull (from Richardson Milling Ltd, Saskatchewan, Canada) was used as adsorbent after it was ground, sieved and dried without further chemical or physical treatment. The raw oat hull was ground and sieved to sizes of 0.425-1.18 mm. After that, the oat hull particles were dried at 105 °C for 48 hr before they were used for adsorption.

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The dehydration of butanol by adsorption using the oat hull based biosorbent is an environmentally friendly and cost-effective process. Low price and environment-friendly are advantages of this adsorbent. Further treatment of the raw oat hull will increase the cost. In addition, the FTIR results of raw oat hull sample in this study were compared to the oat hull sample in another study,36 where the oat hull sample was washed liberally with distilled water for several times. The FTIR results of these two samples are consistent. The FTIR spectra of biosorbent (raw oat hull sample) in this study is largely consistent with the FTIR spectra of three main components (cellulose, hemicellulose and lignin) of oat hull.37 These results confirm that the raw oat hull material used in this study can reasonably represent the pure oat hull. Thus, in this study, the raw oat hull biosorbent after simple treatment was used for experiments. The butanol-water solution was prepared by mixing 1-butanol (ACS reagent grade, Fisher Scientific) with distilled water. 3.2 Characterization of the biosorbent The characterization results from methods such as Fourier transform infrared (FTIR) spectroscopy, Emmett and Teller (BET) surface area, scanning electron microscope (SEM) and thermogravimetric analysis (TGA) of this biosorbent can be found in a previous study.21 The FTIR spectra of the biosorbent before and after adsorption-desorption cycles were obtained by JASCO FT-IR 4100. The particle size distribution of the biosorbent sieved to 0.425-1.18 mm was determined by Mastersizer particle size analyzer. Samples were delivered by the compressed air passing through the measurement area. During measurement, sample particles are passed through the laser beam. The scattering intensity of particles is measured by a photosensitive detector to calculate the particles size. 11



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Coomassie blue staining was conducted to study the water diffusion into the inner structure of the oat hull based biosorbent. Oat hull was stained with Coomassie staining solution (0.25% Coomassie brilliant blue, 50% methanol, 10% acetic acid) for 3 min at room temperature on a shaker to ensure full contact staining. Then the sample was destained in a solution containing 50% methanol and 10% acetic acid for 30 min at room temperature. The stained sample was placed on a glass substrate and covered with a coverslip held in place by double-sided tape along one edge. Finally, imaging was performed using an Olympus BX51M optical microscope with a high-speed camera (Olympus K-TV0.63XC 7J19174) and a high-power illumination source (EXFO X-Cite series 120). Water transport through oat hull was also observed using the Olympus BX51M optical microscope with the high-power illumination source (EXFO X-Cite series 120). Dye solution (1 mL H2O + 1 drop of blue solution) was loaded to the edge of oat hulls sample placed on the glass microscope slide. The photo was taken after 40 min from adding the dye solution. Microscopy images were then analyzed using ImageJ (NIH) software. 3.3 Adsorption and desorption procedure The recovery of biobutanol based on the adsorption method was performed in a fixed bed using a biosorbent based on oat hull. The details of the adsorption system were described in previous studies.21,38 The system contains an adsorption column. The internal diameter of the adsorption column is 4.75 cm, and the length is 0.5 m. The column is equipped with an oil heating jacket to maintain the temperature. In addition to the adsorption column, the adsorption system is composed of a butanol-water feed tank with stirring, a feed pump (Cole-Parmer, RK- 74930-05), a gas flow controller to adjust the flow 12


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rate of the carrier gas, pipelines with heating jacket to evaporate and maintain the butanolwater mixture, two thermocouples (Omega K type, US) to monitor the temperature at the top and bottom of the column, a pressure transducer (Honeywell, US) to monitor the pressure in the column, a back pressure regulator connected to the bottom of the column to maintain stable pressure, and a condenser connected to the outlet of the column to condense the effluent. Experiments were carried out in the column loaded with 300 ± 25 g oat hull based biosorbent (0.425-1.18 mm particle size), particle density is approximately 1440 kg/m3,39 and the bed void fraction is 0.76. In this work, the volume median diameter of particles (approximately 0.92 mm) was determined to present the diameter of the adsorbent for dynamics analysis. The mixtures of butanol and water with different concentrations (range from 56 to 80 wt% butanol) were used as feed to simulate the butanol product generated from preliminary distillation in the butanol production industry. The liquid feed was vaporized by using heating tapes and carried by nitrogen gas at an average superficial velocity of approximately 0.02 m/s. The flow rate was used in this work based on the following considerations: 1) Ensured the water-butanol vapor had sufficient residence time within the column in order for water to be effectively adsorbed by the packed biosorbent, and avoid high pressure drop across the column; 2) Let the column be saturated within a reasonable time period for sample collection. 110 °C and 120 °C were chosen as the temperature of the system to avoid condensation in the column. The pressure of the system was set at 135 kPa to reduce energy consumption. Before each experiment, the system was set to the desired temperature, and then the feed pump was switched on. The liquid feed was pumped and vaporized in order to simulate the low-grade butanol-water vapor generated from the preliminary distillation in the 13



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industrial ABE process. Complete vaporization of the feed was ensured before it was let to the top of the column. The temperature and pressure of the system were monitored by thermocouples and pressure transducer during adsorption experiments. The effluent flowing out of the column was condensed and collected at a time interval. Then, the samples were weighed. The water concentration in effluent samples was analyzed by Karl Fischer Coulometer (Metler Toledo DL32). The regeneration of the biosorbent was carried out under vacuum after adsorption reached equilibrium. For regeneration, nitrogen gas was purged to the column from the bottom under the pressure of 33 kPa at 105 °C to flash out the adsorbate for 240 minutes. Effects of different conditions, i.e., adsorption temperature and feed concentrations were investigated in this work. All the experiments were carried out at least twice, and the results were presented in average and standard deviation for all the data. 4. Results and discussion 4.1 Results of characterization 4.1.1 Particle size distribution The particle size distribution of the oat hull based biosorbent used in this work was analyzed by the Mastersizer particle size analyzer. The volume-based particle size distribution is shown in Figure 1. It can be seen from the results that the size of 60% of the biosorbent particles falls in the range of 0.41-1.19 mm. 34% of them are bigger than 1.19 mm, and 6% of them are smaller than 0.41 mm. The volume median diameter of particles D (v, 0.5) is approximately 0.92 mm. It indicates that 50 % of the particles are bigger than 0.92 mm, and 50% of them are smaller than 0.92 mm.

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Figure 1. Particle size distribution for the biosorbent. 4.1.2 Coomassie blue staining Figure 2 shows the images of oat hull after contacting with Coomassie brilliant blue solution (see materials and methods for details). The images demonstrated that water diffused into the inner part of the oat hull. In certain areas, blue dots and lines were visualized. Oat hull contains cellulose, hemicellulose, lignin and low content of protein. According to the literature,40 Coomassie blue can bind to protein structures through ionic interactions between dye sulfonic acid groups and positive protein amine groups as well as Van der Waals. In addition, Coomassie blue may also attach to cellulose, hemicellulose and lignin structures in oat hull according to lots of studies,41–45 which demonstrated that some cellulose, hemicellulose and lignin based materials have adsorption capacity to adsorb dyes. The detailed investigation in the regards could form an area of future research.

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Figure 2. Coomassie blue stained oat hull (A): imaged using a 20x objective; (B): imaged using a 100x objective (scale bars, 50m). 4.1.3 Water absorption observed by optical microscopy Optical microscopy imaging provides information on the water-dye solution transport through the biosorbent particles (Figure 3). Water diffused into individual oat hull particles, and the adsorption occurred mainly along the longitudinal walls of the hull cells. The dynamic image of the adsorption can be seen in the Supporting Information.

Figure 3. Optical microscopy image of oat hull particles after water adsorption. 4.2 Simulation of water breakthrough curves by kinetics models and comparisons The previous study of the authors reported the results of adsorption capability of water and butanol on the biosorbent, butanol purity, adsorption selectivity of water over butanol, and equilibrium study.21 It demonstrated the affinity between water and oat hull biosorbent was higher than that between butanol and oat hull, and much more water was adsorbed on the oat hull based biosotbent compared with butanol. Thus, the adsorption process was able 16


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to selectively adsorb water over butanol and produce high purity butanol products (up to 99% butanol) from lower grade feed (56%-90% butanol). In addition, water was adsorbed continually throughout the dynamics adsorption process, while butanol adsorption mainly occurred at the beginning of the process and then rapidly decreased to insignificance as a result of the competition of adsorption with water. Considering the adsorption was dominated by water, and the system was complex, this paper focused on the dynamic water adsorption and modeling. Butanol adsorption dynamics will form a separate subject of research. Thus, water adsorption breakthrough curves at different conditions were simulated and discussed in this work. Simulation of water adsorption breakthrough curves by dynamic models can provide valuable information, and assist in evaluating rate controlling steps. The Bohart-Adams model (eq 2) and the Klinkenberg model (eq 3), which are widely used to simulate the breakthrough curves,26,28–30,46 were used to simulate the water breakthrough curves in this work. The suitability of the two models was evaluated in aid of the modeling results, and the effects of operational conditions and mass transfer limitation were discussed. The breakthrough curves for water adsorption on the oat hull based biosorbent at temperatures of 110 °C and 120 °C (feed concentration 56 wt% butanol, pressure 135 kPa) are presented in Figs. 4 and 5. These temperatures were chosen to ensure that water exists in the butanol-water vapor. It shows that the oat hull based biosorbent was able to adsorb water from the butanol-water mixture. At a higher temperature, e.g. 120 °C, the slope of the breakthrough curve is steeper, indicating higher mass transfer rate. In addition, at the beginning of adsorption, the slopes of both the breakthrough curves are steep, which indicates higher mass transfer rate due to the higher water concentration gradient between 17



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biosorbent and vapor feed. When adsorption approaches equilibrium, the slope becomes flatter, indicating a slower mass transfer rate.

Figure 4. The breakthrough curves of water adsorption on the biosorbent (pressure of 135 kPa, 56 wt% butanol feed concentration, superficial velocity of 0.02 m/s) at different temperatures, and simulation curves of the Bohart-Adams model.

Figure 5. The breakthrough curves of water adsorption on the biosorbent (pressure of 135 kPa, 56 wt% butanol feed concentration, superficial velocity of 0.02 m/s) at different temperatures, and simulation curves of the Klinkenberg model. Figures 4 and 5 also show the fitting results of the Bohart-Adams model and Klinkenberg model, respectively. It can be seen from Fig. 4 that the Bohart-Adams model 18


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partially fitted the experimental data. However, it did not fit the data points at the beginning of the breakthrough curves satisfactorily. The values of the parameters kBA and qs were estimated by model fitting. Simulation of the breakthrough curves by the Klinkenberg model is depicted in Fig. 5. The results show that the Klinkenberg model successfully fitted the data points. The curves generated by this model are more consistent with the experimental data than those generated by the Bohart-Adams model. The values of the parameters in the Klinkenberg model were estimated by model fitting as well. The fitting results and the parameters obtained from model fitting are listed in Tables 1 and 2. The model simulation was evaluated by the value of correlation coefficient (R2) and the residual sum of squares (RSS). The values of R2 of the Klinkenberg model are higher than that of Bohart-Adams model, and the values of RSS of the Klinkenberg model are smaller. Comparison of the fitting results shows that the Klinkenberg model is more suitable to simulate the water adsorption from butanol-water mixture than the Bohart-Adams model. Table 1 Fitting results of the Bohart-Adams model at two different temperatures T (°C)

qs (g/cm3)

kBA (cm3g-1min-1)

R2

RSS

110

86.5

0.175

0.90

0.074

120

22.0

0.286

0.90

0.179

Table 2 Fitting results of the Klinkenberg model at two different temperatures T (°C)

K

kLDF (min-1)

R2

RSS

110

316

0.12

0.98

0.029

120

105

0.84

0.98

0.033

The above results suggest that the water adsorption mechanism and dynamics are more likely close to the Klinkenberg model’s theory. The Bohart-Adams model assumes that the diffusion steps are very fast, and the overall adsorption rate is limited by the surface 19



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adsorption step. On the other hand, the Klinkenberg model is based on the mass transfer limitation. The biosorbent is porous, and intrinsic water adsorption on the biosorbent is physical and fast; thus it is reasonable that mass transfer is the rate controlling step instead of surface adsorption. 4.3 Interpretation of the adsorption data of different adsorption temperatures As concluded in the last section, the Klinkenberg model described the data of water adsorption on the biosorbent more satisfactorily. Thus, mass transfer resistance was further analyzed in this section based on the Klinkenberg modeling results. Such information is important as the mass transfer resistance and rate limited step are not clear for water adsorption on the oat hull biosorbent. As described in the section of dynamic models, the overall mass transfer resistance, external, and internal mass transfer resistance can be presented by the first, second, and third terms of eq 6, respectively. Thus, the overall mass transfer resistance (1/kLDFK), external mass transfer resistance (Rp/3kc) and internal mass transfer resistance (Rp2/15De) were estimated using the parameters obtained from the model fitting. The results of mass transfer resistances at various conditions are listed in Table 3. Overall, the values of internal mass transfer resistances are much higher than those of external mass transfer resistance. This finding successfully identified that the water adsorption rate on the biosorbent was mainly controlled by internal mass transfer resistance. This is consistent with the texture of oat hull material, which is porous, as observed through the SEM images reported in a previous publication.21 The similar result was found in the study of ethanol dehydration using canola meal based biosorbent that the internal mass transfer resistances governed the adsorption process.29 From the comparison of the mass transfer at different temperatures, it was confirmed that the water adsorption mass transfer 20


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rate increased with the increase in temperature. This could be due to the higher temperature accelerating diffusion of water molecules. The adsorption equilibrium constant declined with the increase of adsorption temperature, indicating that the water adsorption by the biosorbent in this work is exothermic. The effects of the adsorption temperature on the adsorption capacity for the oat hull based biosorbent have been discussed in a previous study.21 The comprehensive conclusion is consistent with that of the previous work that increase of temperature caused the increase in adsorption rate, though it decreased the adsorption capacity at equilibrium.47 Table 3 Results of mass transfer study at different adsorption temperatures T (°C)

Sh

kc (m/s)

1/kLDFK (s)

Rp/3kc (s)

Rp2/15De (s)

110

3.17

0.064

1.582

2.41×10-3

1.580

120

3.14

0.065

0.680

2.38×10-3

0.678

4.4 Simulation of breakthrough curves for different feed concentrations The breakthrough curves for water adsorption on the oat hull based biosorbent at different feed concentrations (adsorption temperature 110 °C) was further predicted by the Klinkenberg model, as shown in Fig. 6. It can be seen from the experimental data that the feed concentration of adsorbates affected the water adsorption rate. Especially at the beginning of adsorption, the slope of the water breakthrough curve obtained by higher water content in the feed is steeper than that of lower water content in the feed, indicating higher mass transfer rate due to higher mass transfer driving force. Fig. 6 shows that the Klinkenberg model is able to simulate all the breakthrough curves. However, the fitting results in cases of 70 and 80 wt% of butanol concentration in the feed (R2=0.96, 0.95) are not as good as that of 56 wt% of butanol concentration (R2=0.98). This is because that the Klinkenberg model assumed water adsorption isotherm is without the effect of butanol 21



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presence. When butanol concentration in the feed was higher, adsorption of butanol increased, thus such assumption caused deviation of the modeling results from the experimental data. In the tested range of this work, this model provides satisfactory results. To apply this model for an extended range of butanol concentration, it is necessary to consider the effect of butanol in water adsorption isotherm. However, this will bring challenges to determine an analytical solution for the model. This can form an area of future research.

Figure 6. The breakthrough curves of water adsorption on the biosorbent at different feed concentrations (pressure of 135 kPa, temperature of 110 °C, superficial velocity of 0.02 m/s) and the simulation curves of the Klinkenberg model. 4.5 Reusability of the biosorbent In order to investigate the reusability of the oat hull based biosorbent for butanol dehydration, the biosorbent was regenerated and reused in the same column without being changed. After adsorption, nitrogen gas was purged to the column from the bottom under the pressure of 33 kPa at 105 °C to flash out all the adsorbates for 240 minutes. Then, the regenerated bed was reused for adsorbing water from the butanol-water mixture. The 22


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profile of butanol concentration in the effluent for the fresh bed and the regenerated bed (after 20 times adsorption-desorption cycles) at the same conditions are presented in Fig. 7. The butanol concentration of the effluent shows that the oat hull biosorbent is able to concentrate the butanol solution to high purity product. The two curves are overlapped. It was demonstrated that the biosorbent derived from oat hull used in our study was regenerated and reused successfully for more than 20 cycles with satisfactory performance. The FTIR spectra of the biosorbent before and after adsorption-desorption cycles were nearly the same, as shown in Figure 8. The functional groups of the biosorbent did not change after the adsorption-desorption cycles, indicating that the biosorbent has high stability during the adsorption process.

Figure 7. The profile of butanol concentration in effluent on the fresh and regenerated bed at 110 °C, 135 kPa, and 56 wt% butanol feed concentration.

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Figure 8. FTIR spectra of the fresh oat hull biosorbent and FTIR spectra of reused and regenerated oat hull biosorbent. 5. Conclusions In this study, the dynamics of water adsorption on the oat hull based biosorbent was analyzed in aid of mathematical modeling. The Klinkenberg model based on the mass transfer limitation successfully described the experimental water breakthrough curves obtained under the tested conditions. The Bohart-Adams model assuming surface adsorption being the controlling step did not provide satisfactory simulation particularly for the initial part of the breakthrough curves. This is consistent with the facts that the oat hull based biosorbent is a porous material, and water adsorption is physical, thus the overall water adsorption is limited by mass transfer resistance. This implies that the water adsorption mechanism and dynamics are more likely to follow the Klinkenberg model’s theory. In addition, based on the Klinkenberg modeling results, the overall, external, and internal mass transfer resistances of water adsorption on the oat hull-based biosorbent were estimated. The results indicated that the water adsorption rate on the biosorbent was controlled by internal diffusion. From the comparison of the mass transfer at different 24


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temperatures, it was concluded that the water adsorption mass transfer rate increased with the increase in temperature. In addition, water diffusion inside the oat hull was visualized in aid of Coomassie brilliant blue staining and microscope imaging. Furthermore, this work demonstrated that the biosorbent derived from oat hull was viably regenerated and reused for butanol dehydration with good performance and high stability. It is also noted that though the Klinkenberg model provided the satisfactory simulation of water breakthrough curves when butanol content in the feed is in the range of 56-80wt%. For extended applications of the model, it is necessary to consider the effects of butanol presence in the water adsorption. This needs to be done in future investigation. Acknowledgments This work was supported by Mitacs (IT04850), Natural Sciences and Engineering Research Council of Canada (No. RGPIN 299061-2013), Canada Foundation for Innovation (No. 33172), Saskatchewan Ministry of Agriculture and the CanadaSaskatchewan Growing Forward 2-bilateral agreement (No. 20130220), Saskatchewan Canola Development Commission (No. 20130220), and Western Grains Research Foundation (No. 20130220). The authors thank the NanoScience Technology Center (University of Central Florida) for technical assistance in microscope imaging. All the supports are highly appreciated. Supporting Information Supplementary video 1: The dynamic water adsorption on the biosorbent particles captured by optical microscopy imaging.

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